1. Field of the Invention
This invention relates generally to liquid level measuring systems, and more particularly to an improved system employing bubbler tubes having improved tips to prevent variations in the bubble level.
2. Description of the Prior Art
In the prior art a purging bubbler tube system is employed for measuring liquid level. In such a system gas may be periodically allowed to flow through the bubbler tube to purge out any liquid that might have accumulated therein. At the end of the purge the gas is valved off. With the gas valve off the pressure in the bubbler tube will equalize at the head of liquid which is developed at the gas/liquid interface. The gas/liquid interface will be located within or immediately joining the bubbler tube. The pressure within the bubbler tube is measured by some type of instrumentation. The pressure measurement is made between purge periods. In such a purging bubbler tube system the depth of the gas/liquid interface is directly related to the pressure within the bubbler tube. Hence, for consistent pressure readings it is imperative that the gas/liquid interface depth be repeatable and constant for a given liquid level. In the prior art the tip of the bubbler tube in a purging bubbler tube system is generally the horizontal cut at the end of the vertical bubbler tube. The shortcoming of this design is that the gas/liquid interface depth for a given liquid level is not repeatable and constant. The non-repeatability of the gas/liquid interface depth is due to the periodic purge. When the purge valve is closed the depth of the gas/liquid interface is largely determined by the bubbles which the gas forms at the end of the bubbler tube. The gas/liquid interface may be outside of the bubbler tube, a bubble, and sometimes within the bubbler tube after the bubble has just broken off. Other times it will be between these two points. As mentioned previously the variation in gas/liquid interface depth causes a corresponding undesirable variation in the bubbler tube pressure.
Temperature and leaks are the major causes of the gas/liquid interface depth not being constant. Any temperature change of the gas in the bubbler tube after the purged gas is valved off will cause gas to expand or contract, causing the gas/liquid interface to either rise or fall. Any leak, either in the plumbing or the purge valve, will also cause the gas/liquid interface to either rise or fall. Again, this non-constant gas/liquid interface depth will cause unwanted variations in the bubbler tube pressure.
U.S. Pat. No. 4,630,478, Johnson, discloses a liquid volume sensor system employing two bubbler tubes which are bent at substantially a right angle at their ends in order to contain a cylinder 5 between the tips. Expansion of the cylinder 5 is used for temperature compensation by changing the spacing between the tips of the tubes. Johnson contains no disclosure about the desired length of the bend and does not show any slanted cutoff at the tips of the tubes. He does not address the problem of the present invention. There is no disclosure of the length of the bends being sufficient to solve the problem of the bubble backing up in the tube.
Of the patents cited in Johnson only U.S. Pat. No. 3,473,379 shows a right angle bend in the end of the pressure tube. However, the pressure sensors in this patent employ transducers in the tube and it is not a bubbler system.
Sci-Tec Instruments, Incorporated of Saskatoon, Canada makes a device called "Petro Tag," a hydrostatic system which uses a Teflon coated disk at the end of the bubbler tube to trap a big bubble to try to solve the same problem solved by the present invention.
Accordingly, it is an object of the present invention to provide a bubbler tube tip design to maintain the gas/liquid interface in a bubbler tube level measuring system at a constant depth.
Another object is to provide such a device in which any backing up of the bubble in the tube leaves the bubble at a constant depth.
Still another object is to provide a tip for the bubbler tube designed to prevent the bubble from sagging at the end of the tube, again to maintain a constant depth.